For power field effect transistors (FETs) lower specific ON-resistance (RSP) is generally the trend for improving the performance including raising the power conversion efficiency. One such power device is a planar gate trench metal oxide semiconductor FET, such as the Texas Instruments' NEXFET™.
The NEXFET™ is derived from a structure resembling a laterally diffused metal oxide semiconductor (LDMOS) transistor that can be used for RF signal amplification in a frequency range up to about 2 GHz. In an LDMOS device, the drain is laterally arranged to allow current to laterally flow, and a drift region is interposed between the channel and the drain to provide a high drain to source breakdown voltage.
For the NEXFET™ to achieve higher dynamic performance, the Miller capacitance is reduced to single picofarads by having the overlap of the gate electrode over the lightly doped drain extension (LDD) kept to a minimum. Additionally, the NEXFET™ has a topology with the source metal wrapping the gate electrode and creating a field-plate element over the LDD region making an effective electrostatic shield between the gate and drain terminals of the device. The field-plate created by the source metal also stretches out the distribution of the electric field along the LDD surface. This field plate feature lowers the height of the electric field peak at the drain corner of the gate electrode. By doing so, the hot-carrier effects creating reliability issues in conventional LDMOS devices are avoided.